Biosorption of chromium and zinc ions by an industrial algal waste, from agar extraction industry has been studied in a batch system. This biosorbent was compared with the algae Gelidium itself, which is the raw material for agar extraction, and the industrial waste immobilized with polyacrylonitrile (composite material). Langmuir and Langmuir-Freundlich equilibrium models describe well the equilibrium data. The parameters of Langmuir equilibrium model at pH 5.3 and 20 degrees C were for the algae, q(L)=18 mg Cr(III) g(-1) and 13 mgZn(II) g(-1), K-L=0.0211 mg(-1) Cr(III) and 0.0261 mg(-1) Zn(II); for the algal waste, q(L)=12 mg Cr(III) g(-1) and 7 mg Zn(II) g(-1), K-L=0.0331 mg(-1) Cr(III) and 0.0421 mg(-1) Zn(II); for the composite material, q(L)=9 mg Cr(III) g(-1) and 6 mg Zn(II)g(-1), K-L=0.0321 mg(-1) Cr(III) and 0.0341 mg(-1) Zn(II). The biosorbents exhibited a higher preference for Cr(III) ions and algae Gelidium is the best one. The pseudo-first-order Lagergren and pseudo-second-order models fitted well the kinetic data for the two metal ions. Kinetic constants and equilibrium uptake concentrations given by the pseudo-second-order model for an initial Cr(III) and Zn(II) concentration of approximately 100 mg l(-1), at pH 5.3 and 20 degrees C were k(2,ads) =0.04 g mg(-1) Cr(III) min(-1) and 0.07 g mg(-1) Zn(II) min(-1), q(eq)=11.9 mg Cr(III) g(-1) and 9.5 mg Zn(II) g(-1) for algae; k(2,ads) =0.17 g mg(-1) Cr(III) min(-1) and 0.19 g mg(-1) Zn(II) min(-1), q(eq)=8.3 mg Cr(III) g(-1) and 5.6 mg Zn(II) g(-1) for algal waste; k(2,ads)=0.01 g mg(-1) Cr(III) min(-1) and 0.18 g mg(-1) Zn(II) min(-1), q(eq)=8.0 mg Cr(III) g(-1) and 4.4 mg Zn(II) g(-1) for composite material. Biosorption was modelled using a batch adsorber mass transfer kinetic model, which successfully predicts Cr(III) and Zn(H) concentration profiles. The calculated average homogeneous diffusivities, D-h, were 4.2 x 10(-8), 8.3 x 10(-8) and 1.4 x 10(-8) cm(2) s(-1) for Cr(III) and 4.8 x 10(-8), 9.7 x 10(-8) and 6.2 x 10(-1) cm(2) s(-1) for Zn(II), respectively, for Gelidium, algal waste and composite material. The algal waste has the lower intraparticle resistance. (C) 2007 Elsevier B.V. All rights reserved.